WO2015124653A1

WO2015124653A1 - Facility and method for the high-speed heat treatment of a steel reinforcement element for tyres

Info

Publication number: WO2015124653A1
Application number: PCT/EP2015/053456
Authority: WO
Inventors: Pierre MORISOT; Elisabeth FAUCHEUX; Benoît RENAUX; Eric Colin
Original assignee: Compagnie Generale Des Etablissements Michelin; Michelin Recherche Et Technique S.A.
Priority date: 2014-02-21
Filing date: 2015-02-19
Publication date: 2015-08-27
Also published as: FR3017881A1

Abstract

The invention relates to a facility for the heat treatment of a steel reinforcement element (F) for tyres, comprising a device for heating the reinforcement element (F) to a temperature higher than, or equal to, the austenitisation temperature of the steel, comprising means (21) for supplying calories by means of the Joule effect through the reinforcement element (F).

Description

Installation and method for high speed heat treatment of a steel reinforcing element for a tire.

[001] The invention relates to an installation and a method of heat treatment of a steel reinforcing element for a tire.

[002] It is known from the state of the art a method of manufacturing a steel reinforcing element for a tire, for example a steel wire.

[003] The manufacturing method allows to manufacture, from a wire, said wire rod, of initial diameter between 4.5 and 7.5 mm, a wire used for reinforcing plies of the tire of diameter between 0.08 mm and 0.50 mm.

[004] Firstly, the predominantly pearlitic microstructure wire is drawn, for example in a dry medium, so as to reduce its initial diameter to an intermediate diameter, for example equal to 1.3 mm. At the end of this drawing step, the steel of the wire has a microstructure comprising several mixed phases.

Then, the intermediate diameter wire is thermally treated so as to modify the microstructure of the steel. In this case, the predominantly pearlitic microstructure of the steel is regenerated.

After having coated the wire of intermediate diameter with a metal layer, the wire of intermediate diameter coated is drawn, for example in a wet medium, so as to reduce its diameter to a final diameter, for example equal to 0.20. mm.

[007] US4767472 discloses a method of heat treatment of the intermediate diameter wire comprising three steps and carried out by means of a heat treatment plant.

[008] The heat treatment plant comprises, in the direction of travel of the wire, upstream means for storing the untreated wire, for example upstream coils, a heating device, a cooling device and means for downstream storage. treated wire, for example downstream coils.

In a first step, the wire temperature is raised beyond the austenitization temperature of the steel to obtain a predominantly austenitic microstructure. For this purpose, the heat treatment plant comprises a wire heating device comprising a gas oven.

[010] Then, in a second step, the temperature of the wire is lowered to obtain a metastable austenitic microstructure by means of a cooling device comprising a water bath. The bath comprises pure liquid water at a temperature above 80 ° C in which the wire is scrolled. [011] In a third step performed downstream of the bath, the temperature of the wire is allowed to lower to the ambient air or in a thermally insulated device. During this exposure to ambient air, the predominantly austenitic microstructure is converted into a predominantly pearlitic microstructure or else this transformation initiated previously in the bath is continued through the pearlitic transformation domain.

[012] However, the gas furnace used to increase the temperature of the wire is energetically inefficient and very energy-intensive. Thus, to increase the temperature of the wire to a temperature greater than or equal to the austenitization temperature of the steel, the wires are run at a relatively low speed of less than or equal to 40 m. min ^"1. Indeed, too fast frame rate would lead to insufficient heating and to obtain a different microstructure predominantly austenitic microstructure desired.

The object of the invention is an installation and a method making it possible to obtain a high unit mass flow rate.

[014] For this purpose, the subject of the invention is a heat treatment installation of a steel reinforcing element for a tire comprising a device for heating the reinforcing element at a temperature greater than or equal to the temperature of austenitization of the steel comprising means for supplying calories by joule effect through the reinforcing element.

[015] The means for supplying calories by Joule effect allow a direct supply of calories to the reinforcing element through the latter. Thus, the temperature is very rapidly increased, which makes it possible to use a high scrolling speed and thus to obtain a high unit mass flow rate. Indeed, the intake of calories by joule effect is extremely effective because achieved through the wire without convection.

[016] In addition, the means for supplying calories Joule effect result in a relatively low energy expenditure and controlled compared to other means of supply of calories, including convection type such as gas ovens.

[017] In the present description, any range of values designated by the expression "between a and b" represents the range of values from more than a to less than b (i.e., terminals a and b excluded ) while any range of values designated by the expression "from a to b" signifies the range of values from a to b (that is to say, including the strict limits a and b).

[018] In one embodiment, the Joule effect calorie delivery means comprise two electrically conductive terminals. [019] In one embodiment, each electrically conductive terminal comprises an electrically conductive rotary pulley.

[020] The rotary pulleys allow both the electrical conduction and the passage and the guide of the reinforcing element, and whatever the speed of travel of the latter.

[021] In an embodiment, downstream of the heating device, a cooling device of the reinforcing element.

[022] Preferably, the cooling device comprises means for extracting calories from the reinforcing element.

[023] Advantageously, the means for extracting calories comprise:

a scrolling chamber of the reinforcing element containing an intermediate cold source arranged between the reinforcing element and an external cold source,

a circulation chamber of the external cold source arranged around the scroll chamber of the reinforcing element.

[024] Such extraction means are compatible with a high scrolling speed. Indeed, the calorie extraction means have a heat removal capacity significantly greater than the water bath used in the state of the art which allows to increase the speed of travel of the reinforcing element.

[025] Moreover, a high speed in the bath used in the state of the art leads to a turbulent flow of water in contact with the wire and therefore to an insufficient and poorly controlled lowering of the temperature of the wire, which then causes the appearance of defects on the surface of the wire. By removing the water bath, it eliminates the problem of the appearance of turbulent flow at high speed and thus the appearance of defects on the surface of the reinforcing element.

[026] In addition, the extraction means are much safer than the baths usually used, whether water bath, lead or molten salts. In fact, the circulation chamber makes it possible to physically isolate the reinforcement element from the operators.

[027] Unlike baths of lead or molten salts that can pose environmental and safety problems, the extraction means are safe and environmentally friendly. In addition, the calorie extraction means used make it possible to avoid any cleaning step designed to remove lead or molten salts covering the reinforcing element. [028] Preferably, the intermediate cold source comprises a heat exchange gas. In one embodiment, the heat exchange gas may comprise one or more gaseous constituents.

[029] Preferably, the external cold source comprises a heat exchange liquid.

Another object of the invention is a method of heat treatment of a steel reinforcing element for a tire comprising a step of increasing the temperature of the reinforcing element at a temperature greater than or equal to the temperature of the reinforcing element. austenitizing during at least a portion of which calories are supplied to the reinforcing element by joule effect through the reinforcing element.

[031] The invention applies to any type of heat treatment process requiring a temperature increase of the reinforcing element at a temperature greater than or equal to the austenitization temperature of the steel constituting it. Thus, the process according to the invention can be applied to a continuous cooling thermal treatment process (abbreviated as "TTRC") as well as to an isothermal heat treatment process (abbreviated as "TTI").

[032] Advantageously, it scrolls the reinforcing element at an average speed of scrolling strictly greater than 40 m. min ^-1 , preferably greater than 90 m ^-1 min ^-1 , more preferably greater than or equal to 200 m. min ^-1 and even more preferably greater than or equal to 300 m min ^-1 .

[033] By average speed, it is necessary to understand the ratio of the distance traveled by a point of the reinforcing element on the time taken by this point to travel this distance.

[034] Unlike the process described in US Pat. No. 4,767,472 and methods of the TTI type necessarily requiring a relatively long transformation time, for example of the order of several ten seconds, the transformation in a TTRC type process can be relatively short, by example of the order of a few seconds, which allows to use high scroll speeds in a facility with a small footprint.

[035] Advantageously, the temperature of the reinforcement element is increased at an average rate of increase ranging from 100 to 1000 ° Cs ^-1 , preferably 500 to

950 ° Cs ^"1 and more preferably 700 to 900 ° Cs ^{" 1} .

[036] Preferably, the method comprises a step of lowering the temperature of the reinforcement element by continuous cooling:

from an initial temperature of an initial stability domain of the steel,

up to a final temperature of a final stability domain of the steel, the step of lowering the temperature comprising a transformation of the microstructure of the steel from a microstructure of the initial domain to a microstructure of the final domain.

[037] Thus, the method is relatively robust and simple to control. Indeed, unlike the method described in US4767472 and certain heat treatment processes of the state of the art, called isothermal transformation (abbreviated "TTI" for isothermal heat treatment), in which the transformation of the steel takes place at a substantially constant temperature, the process is here continuous cooling (abbreviated "TTRC" for heat treatment with continuous cooling). The two types of process are easily distinguished, in particular thanks to the time-temperature diagrams used to represent them. A TTI type method uses a TTT (time-temperature-transformation) diagram and includes one or more changes in speed during the temperature lowering step. A TTRC method uses a TRC chart and has a continuous rate during the temperature lowering step. Thus, among other characteristics of a TTRC type process, the temperature of the reinforcing element is lowered during the transformation of the microstructure of the steel.

[038] In the described method, once the lowering speed of the defined temperature, it is then relatively easy to control it in view of its continuity during the lowering temperature step.

[039] In addition, unlike TTI type processes in which a very high number of calories is provided to the reinforcing element to maintain it at a substantially constant temperature during the transformation, the TTRC type process makes it possible to reduce the energy consumption of the process.

[040] In one embodiment, the average temperature lowering rate during the transformation of the steel microstructure is greater than or equal to 30 ° Cs ^-1 , preferably 50 ° Cs ^-1 and more preferably at 70 ° Cs ^"1 .

[041] The use of an average lowering speed too low does not allow to quickly perform the transformation of the microstructure of the steel. Thus, the risk of obtaining a steel having undesired mechanical properties is minimized.

[042] In one embodiment, the average rate of lowering of the temperature during the transformation of the microstructure of the steel is less than or equal to 110 ° Cs ^-1 , preferably 100 ° Cs ^-1 and more preferably at 90 ° Cs ^"1 . [043] The use of a too high lowering speed has the risk of leading to a hardening of the steel which, according to the desired properties of the steel, is undesirable.

[044] By mean speed of lowering is meant the ratio of the difference in degrees Celsius between the temperature before transformation and after transformation on the time taken to perform the transformation.

[045] Thus, one may consider several embodiments in which the average lowering speed is within intervals of 30 ° Cs ^"1 to 90 ° ^{Cs' 1,} of 30 ° ^{Cs" 1} at 100 ° Cs ^{" 1,} of 30 ° Cs ^"1 at 110 ° ^{Cs" 1,} of 50 ° Cs ^"1 to 90 ° ^{Cs' 1,} of 50 ° ^{Cs" 1} at 100 ° Cs ^"1, of 50 ° ^{Cs" 1} to 1 10 ° Cs ^"1 , 70 ° Cs ^{" 1} at 90 ° Cs ^"1 , 70 ° Cs ^{" 1} at 100 ° Cs ^"1 and 70 ° Cs ^{" 1} at 110 ° Cs ^"1 .

[046] In one embodiment, the initial temperature is greater than or equal to 750 ° C, preferably 800 ° C and more preferably 850 ° C.

[047] In one embodiment, the final temperature is less than or equal to 650 ° C, preferably at 550 ° C and more preferably at 450 ° C.

[048] Thus, it will be possible to envisage several embodiments in which the initial temperature / final temperature pairs are 750 ° C / 450 ° C, 750 ° C / 550 ° C, 750 ° C / 650 ° C, 800 ° C / 450 ° C, 800 ° C / 550 ° C, 800 ° C / 650 ° C, 850 ° C / 450 ° C, 850 ° C / 550 ° C, 850 ° C / 650 ° C.

[049] In one embodiment, the transformation of the microstructure of the steel takes place in a temperature range from 800 ° C. to 400 ° C., preferably from 750 ° C. to 500 ° C. and more preferably from 750 ° C. ° C at 600 ° C.

[050] Advantageously, the temperature is lowered by more than 30 ° C., preferably by more than 50 ° C., more preferably by more than 75 ° C. and even more preferentially by more than 100 ° C. during the transformation of the microstructure of the polymer. 'steel.

[051] The expression "more than X ° C" means that the temperature is lowered by a thermal amplitude strictly greater than X ° C, the value of X ° C being excluded.

[052] In one embodiment, the initial domain is the austenitic stability domain of the steel. Thus, the steel preferably has an initial predominantly austenitic microstructure.

[053] In one embodiment, the final domain is the pearlitic stability domain of steel. Thus, in this area of pearlitic stability, the steel has a final predominantly pearlitic microstructure. Steel can also include, depending on its composition and the various process parameters, bainite and / or ferrite and / or martensite. In the case where one or more of these microstructures are present, the pearlitic microstructure is preferably predominant.

[054] By predominantly austenitic / pearlitic microstructure is meant, in a manner known to those skilled in the art, that the steel comprises a majority austenitic / pearlitic phase relative to the other phases that make up the steel. By major phase, it is understood that the steel comprises, by mass, a higher percentage of this phase compared to the sum of the percentages of the other phases, that is to say more than 50%, preferably more than 75% or even more than 95% and more preferably more than 98% by weight of this phase relative to the sum of the percentages of the other phases.

[055] Advantageously, the step of lowering the temperature comprises a lowering of the temperature of the reinforcing element in the initial stability range of the steel.

[056] Advantageously, the step of lowering the temperature comprises a lowering of the temperature of the reinforcing element in the final stability range of the steel.

[057] In one embodiment, the microstructure of the steel is transformed by traversing at least one transformation domain.

[058] Advantageously, the inlet temperature in the transformation domain, that is to say the temperature delimiting the passage between the initial stability range and the transformation domain, is greater than or equal to 600 ° C. preferably at 650 ° C and more preferably at 700 ° C.

[059] Advantageously, the outlet temperature of the transformation domain, that is to say the temperature delimiting the passage between the transformation domain and the final stability range, is greater than or equal to 400 ° C., preferably greater than or equal to 400 ° C. 500 ° C and more preferably at 600 ° C.

[060] In a preferred embodiment, the transformation domain comprises the ferritic transformation domain.

[061] In a preferred embodiment, the transformation domain comprises the pearlitic transformation domain.

[062] In one embodiment, prior to the step of lowering the temperature of the reinforcing element, the method comprises a step of increasing the temperature of the reinforcing element to a temperature greater than or equal to at the austenitization temperature of the steel. [063] By heating the steel beyond the austenitization temperature of the steel, a predominantly austenitic or even totally austenitic microstructure is obtained.

[064] Optionally, calories are supplied to the reinforcing member during at least a portion of the step of lowering the temperature of the reinforcing member.

[065] The supply of calories to the reinforcing element makes it possible to control the lowering of the temperature of the reinforcing element and thus to obtain the desired microstructure of the steel. In particular, the contribution of calories makes it possible not to lower the temperature too quickly, which would lead to the creation of unwanted phases in the microstructure of the steel.

[066] Contrary to the calorie intake of a TTI type process, the input of calories from the process is relatively low because it is not intended to maintain the temperature of the reinforcing element constant.

[067] According to other optional features of the method:

[068] - The reinforcing element made of steel is a steel wire.

[069] - The steel wire has a diameter ranging from 0.5 to 5.5 mm, preferably from 0.5 to 3 mm and more preferably from 1 to 2.5 mm.

[070] - The steel comprises from 0.4% to 1, 2%, preferably from 0.4% to 1% and more preferably from 0.4% to 0.8% by weight of carbon.

[071] Another object of the invention is a steel reinforcing element obtainable by a method as described above.

[072] The invention will be better understood on reading the description which follows, given solely by way of non-limiting example and with reference to the drawings in which:

FIG. 1 is a diagram of a heat treatment installation according to a first embodiment of the invention for implementing the method according to the invention;

Figure 2 is a diagram of a heating device of the installation of Figure 1;

FIG. 3 is a diagram of a device for maintaining the temperature of the installation of FIG. 1;

Figure 4 is a diagram of a cooling device of the installation of Figure 1;

Figure 5 is a sectional view along VV of the cooling device of Figure 4; FIG. 6 is a diagram illustrating various steps of a method of manufacturing a reinforcing element comprising steps of a thermal treatment method according to the invention;

Fig. 7 is a time-temperature diagram TRC illustrating the heat treatment process of Fig. 6; and

Figure 8 is a diagram of a heat treatment plant according to a second embodiment of the invention.

[073] EXAMPLE OF A THERMAL TREATMENT PLANT ACCORDING TO THE INVENTION FOR THE IMPLEMENTATION OF THE PROCESS ACCORDING TO THE INVENTION

[074] There is shown in Figure 1 a first embodiment of a heat treatment plant of a tire reinforcing element, designated by the general reference 10.

[075] The processing installation 10 is able to process reinforcing elements F made of steel, here steel wires. The steel wires F have a diameter ranging from 0.5 to 5.5 mm, preferably from 0.7 to 3 mm and more preferably from 1 to 2.5 mm.

[076] The installation 10 comprises, in the direction of the scrolling of the element F in the installation 10, from upstream to downstream, means 12 for upstream storage of the element F, a device 14 for heating the element F, a device 15 for maintaining the temperature of the element F, a device 16 for cooling the element F and means 18 for storage of the element F heat-treated.

[077] The upstream means 12 and downstream 18 storage each comprise a storage coil of the element F respectively for unwinding and winding the element F. In the upstream storage means 12, the element F is then connected to the potential P0.

[078] There is shown in Figure 2 the heating device 14. The device 14 is used to heat the element F at a temperature greater than or equal to the austenitization temperature of the steel.

[079] The heating device 14 comprises means 20 for supplying calories to the element F. The calorie supply means 20 comprise means for supplying calories 21 by joule effect through the element F These calorie supply means 21 comprise two electrically conductive terminals 22, 24 fed by a current source, here a transformer 26. Each terminal 22, 24 is respectively connected to the phase P1 and to the neutral N1. In this case, each terminal 22, 24 respectively comprises an electrically rotating pulley 23, 25 conductive. Each terminal 22, 24 is arranged such that each pulley 23, 25 is in contact with the element F during operation of the installation 10.

[080] FIG. 3 shows the device 15 for maintaining the temperature. The device 15 is arranged between the heating device 14 and the cooling device 16 and makes it possible to maintain the temperature of the element F at a temperature greater than or equal to the austenitization temperature of the steel.

[081] The holding device 15 comprises means 31 for supplying calories to the element F. The calorie supply means 31 comprise calorie supply means 33 by Joule effect through the element F These calorie supply means 31 comprise two electrically conductive terminals 35, 37 fed by a current source, here a transformer 39. Each terminal 35, 37 is respectively connected to the phase P2 and the neutral N2. In this case, each terminal 35, 37 comprises respectively a pulley 41, 43 rotating electrically conductive. Each terminal 35, 37 is arranged such that each pulley 41, 43 is in contact with the element F during operation of the installation 10.

[082] The installation 10 also comprises means 49 for biasing the element F against the terminals 24, 35 and more precisely in contact with the pulleys 25, 41. The biasing means 49 here comprise a rotary pulley 51.

[083] FIGS. 4 and 5 show the cooling device 16.

[084] The cooling device 16 comprises means 30 for supplying calories to the element F and means 32 for extracting calories from the element F to at least one cold source, here two cold sources. The cold source or sources are distinct from the ambient air.

[085] The calorie supply means 30 comprise means for supplying calories 34 by the Joule effect through the element F. The calorie supply means 34 comprise two electrically conductive terminals 36, 38 disposed respectively in upstream and downstream of an inlet 40 and an outlet 42 of the element F of the cooling device 16. Each terminal 36, 38 is respectively connected to the phase P3 and to the neutral N3, the neutral N3 being at the same potential T. Each terminal 36, 38 respectively comprises an electrically conductive rotary pulley 53, 55. The calorie supply means 34 also comprise a source of current, here a transformer 44, supplying the two terminals 36, 38. Each terminal 36, 38 is arranged such that each pulley 53, 55 is in contact with the element F during operation of the installation 10. [086] The installation 10 also comprises means 61 for biasing the element F against the terminals 37, 36 and more precisely in contact with the pulleys 43, 53. The biasing means 61 here comprise a rotary pulley 63.

[087] The calorie extraction means 32 are of the convective exchange type between the element F and the cold source or sources.

[088] The calorie extraction means 32 comprise an enclosure 46 for scrolling the element F. The scroll enclosure 46 forms a liner and has a generally symmetrical shape with respect to the axis X scrolling element F, in this case circular general section. The scroll chamber 46 contains an intermediate cold source 48. The heat exchange between the element F and the intermediate cold source 48 is by convection, here by forced convection because of the circulation of the intermediate cold source 48 in the scrolling chamber 46 from upstream to downstream in the direction of movement of the element F. Alternatively, the cold source is devoid of forced convection. For example, the intermediate cold source 48 comprises a heat exchange gas 50. Preferably, the gas 50 is selected from reducing gases, neutral gases and mixtures of these gases. More preferably, the gas 50 is a reducing gas, here dihydrogen H ₂ . In a variant, the gas 50 comprises several gaseous constituents, for example an H ₂ + N ₂ mixture. In another variant, the gas 50 is nitrogen N ₂ .

[089] The calorie extraction means 32 also comprise a chamber 52 for circulation of an external cold source 54. The circulation chamber 52 forms a jacket and has a general shape with axial symmetry with respect to the X axis scrolling element F, in this case circular general section. The circulation chamber 52 is radially external with respect to the scroll chamber 46 and arranged around the latter. The intermediate cold source 48 is arranged between the element F and the external cold source 54. The heat exchange between the intermediate cold source 48 and the external cold source 54 is by convection, here by forced convection due to the circulation. the external cold source 54 in the circulation chamber 52, downstream upstream in the direction of movement of the element F.

[090] For example, the external cold source 54 comprises a liquid 56 of heat exchange, here water.

[091] The calorie supply means 30 and calorie extraction means 32 are arranged so that the temperature of the element F at the outlet 42 is strictly lower than the temperature of the element F at the inlet 40 . [092] EXAMPLE OF THERMAL TREATMENT PROCESS ACCORDING TO THE INVENTION

[093] We will now describe an example heat treatment process of the steel reinforcing element F for tire with reference to Figures 6 and 7 in the context of a method of manufacturing the element F.

[094] The steel comprises for example from 0.4% to 1, 2%, preferably from 0.4% to 1% and more preferably from 0.4% to 0.8% of carbon by weight. The steel may also include specific addition elements such as Cr, Ni, Co, V, or various other known elements (see, for example, Research Disclosure 34984 - "Micro-alloyed steel cord constructions for tires" - May 1993; Disclosure 34054 - "High tensile strength steel cord constructions for tires" - August 1992). In this case, a conventional steel with 0.7% carbon is used.

[095] Prior to the heat treatment process, the element F is drawn, for example, in a dry medium, so as to reduce its initial diameter equal to 5.5 mm to an intermediate diameter, here equal to 1.3 mm in a step 100. At the end of this drawing step 100, the steel of the element F has a microstructure comprising several mixed phases.

[096] The heat treatment process is then implemented in which the intermediate diameter element F is heat-treated in order to modify the microstructure of the steel. In this case, the predominantly pearlitic microstructure is regenerated.

[097] The heat treatment process comprises a step 200 of increasing the temperature of the element F from a temperature T0 to a temperature T1 greater than or equal to the austenitization temperature of the steel. As illustrated in FIG. 7, this step lasts from t0 to t1 and is implemented thanks to the heating device 14.

During at least part of this step 200, calories are supplied to the element F by the Joule effect through the element F. Each terminal 22, 24 is in contact with the element F during of this step 200.

[099] The temperature of the element F is increased from T0 to T1 at an average rate of increase ranging from 100 to 1000 ° C.s ^-1 , preferably 500 to 950 ° C.sup.- ¹ and more preferably from 700 to 100.degree. 900 ° Cs ^-1 Here, the rate of increase is 836 ° Cs ^-1 .

Preferably, T0 is less than or equal to 100 ° C and more preferably at 50 ° C. Here, T0 = 20 ° C. Preferably, T1 is greater than or equal to 850 ° C and more preferably 900 ° C. Here, T1 = 975 ° C. The method comprises a step 202 of maintaining the temperature of the element F at a temperature greater than or equal to the temperature T1. As illustrated in FIG. 7, this step 202 lasts from t1 to t2 and is implemented thanks to the temperature-maintaining device 15.

Alternatively, the method may not include step 202 for maintaining the temperature of the steel at the austenitization temperature of the steel. Thus, it will be possible to envisage a method without a temperature maintenance step, that is to say in which the steps of increasing the temperature of the element F from a temperature T0 to a higher temperature T1 are carried out consecutively. or equal to the austenitizing temperature of the steel and lowering the temperature described below. In this variant, the installation 10 does not include a device 15 arranged between the heating device 14 and the cooling device 16.

Then, the element F arrives at the input 40 of the calorie extraction means 32 of the device 16 at time t2. The method then comprises a step 204 of lowering the temperature of the element F by continuous cooling from an initial temperature T2 to a final temperature T3. As illustrated in FIG. 7, this step 204 lasts from t2 to t3 and is implemented thanks to the cooling device 16.

To lower the temperature from T2 to T3, calories are extracted from the element F by thermal convection in contact with the intermediate cold source 48, in the example thanks to the calorie extraction means 32.

The temperature is lowered from T2 to T3 at an average lowering rate greater than or equal to 30 ° Cs ^-1 , preferably 50 ° Cs ^-1 and more preferably 70 ° Cs ^-1 . preferred embodiment, the lowering rate is less than or equal to 110 ° Cs ^-1 , preferably 100 ° Cs ^-1 and more preferably 90 ° Cs ^-1 .

In the example described, T2 is greater than or equal to 750 ° C, preferably 800 ° C and more preferably 850 ° C. Here, T2 = T1 = 975 ° C.

In the example described, T3 is less than or equal to 650 ° C, preferably at 550 ° C and more preferably at 450 ° C. Here T3 = 400 ° C.

[0108] Preferably, calories are provided to the element F during at least part of the lowering step 204, in the example by the means of supplying calories 34 by the Joule effect through the element F which is then in contact with terminals 36, 38.

The temperature lowering step 204 is subsequent to the temperature maintenance step 202 which is itself subsequent to the step 200 increase in temperature.

The element F is scrolled at an average speed of scrolling preferably strictly greater than 40 m. min ^"1, preferably strictly greater than 90 ^m.min" ^1, more preferably greater than or equal to 200 m.min ^"1 and more preferably greater than or equal to 300 ^{m.min" 1.} Here, the scroll speed is equal to 315 m.min ^"1 .

The inlet temperature of the water is between 20 ° C and 40 ° C, and here substantially equal to 15 ° C. The inlet temperature of the gas is substantially equal to 20 ° C.

The element F then arrives at the outlet 42 of the calorie extraction means 32 of the device 16 at time t3. The element F obtained by the process has in this example a breaking strength Rm equal to 1150 MPa.

Subsequent to the heat treatment process, in a step 300, the thermally treated intermediate-diameter element F is coated with a metal layer, for example a brass layer.

Then, in a step 400, is drawn, for example, in a wet medium element F of heat-treated intermediate diameter coated so as to reduce its diameter to a final diameter, for example equal to 0.23 mm.

The element F thus obtained can be used as a unitary wire for the reinforcement of tire plies or for the manufacture of a layered cable or of a stranded cable for the reinforcement of tire plies. .

The heat treatment process in FIG. 7 is illustrated by a curve C representing the variation of the temperature of the element F as a function of time. We will describe the lowering step 204 with reference to FIG. 7.

The initial temperature T2 belongs to a range of initial stability of the steel, here the austenitic stability domain I in which the steel has a predominantly austenitic microstructure.

The final temperature T3 belongs to a final stability domain of steel, here the pearlitic stability domain IV in which the steel has a predominantly pearlitic microstructure.

The lowering step 204 comprises a lowering of the temperature of the element F in the initial stability range, here in the austenitic stability domain. This lowering is illustrated by the part C1 of the curve C between the points (t2, T2) and (t2 ', T2'). Then, the lowering step 204 comprises a transformation of the microstructure of the steel from the microstructure of the initial domain, here austenitic, to a microstructure of the final domain, here pearlitic. The microstructure of the steel is transformed by traversing at least one transformation domain. In this case, one crosses successively the domains of ferritic transformation II (part C2 of the curve C) and pearlitic III (part C3 of the curve C) between the points (t2 ', T2') and (t3 \ T3 ') .

This transformation of the microstructure of the steel takes place in a temperature range [Τ2 ', Τ3'] ranging from 800 ° C. to 400 ° C., preferably from 750 ° C. to 500 ° C. and more preferably from 750 ° C to 600 ° C. The inlet temperature T2 'in the ferritic transformation domain II, that is to say the temperature delimiting the passage between the initial stability range I and the ferritic transformation domain II, is greater than or equal to 600 ° C. preferably at 650 ° C and more preferably at 700 ° C. The output temperature T3 'of the pearlitic transformation domain III, that is to say the temperature delimiting the passage between the pearlitic transformation domain III and the final stability domain IV, is greater than or equal to 400 ° C. preferably at 500 ° C and more preferably at 600 ° C. In this case, T2 '= 710 ° C and T3' = 600 ° C

During this transformation of the microstructure of the steel, the temperature of the element F is lowered, simultaneously by extracting calories from the element F by thermal convection in contact with the intermediate cold source 48 and providing calories. to the element F joule effect through the element F. In order to lower the temperature of the element F, it extracts more calories than it brings.

During this transformation of the microstructure of the steel, the temperature of the element F is lowered, for example by more than 30 ° C., preferably more than 50 ° C., more preferably more than 75 ° C., and more preferably more than 100 ° C. In this case, the temperature is lowered to 123 ° C.

During this transformation, the average rate of lowering of the temperature is greater than or equal to 30 ° Cs ^-1 , preferably 50 ° Cs ^"1 and more preferably 70 ° Cs ^{" 1} .

During this transformation, the average rate of lowering of the temperature is less than or equal to 110 ° Cs ^-1 , preferably 100 ° Cs ^-1 and more preferably 90 ° Cs ^"1 .

In this case, the average rate of lowering of the temperature is equal to 86 ° Cs ^"1 . Then, the lowering step 204 comprises a lowering of the temperature of the element F in the final stability range, here the perlitic stability domain. This lowering is illustrated by the part C4 of the curve C between the points (t3 \ T3 ') and (t3, T3).

It will be noted that the method described is continuous cooling. Thus, as illustrated in the TRC diagram of FIG. 7, there is no sudden change in the cooling rate of the element F between the temperatures T2 and T3. Further, the transformation takes place entirely between the inlet 40 and the outlet 42 of the cooling device 16. In other words, the element F leaves the initial stability domain downstream of the inlet 40 and upstream of the the output 42 and reaches the final stability range downstream of the input 40 and upstream of the output 42.

By transforming the microstructure of the steel between the inlet 40 and the outlet 42 of the cooling device 16, it limits or even avoids the formation of oxides on the surface of the steel contrary to a heat treatment taking place in whole or in part in an oxidizing medium, for example in ambient air.

The skilled person is able to distinguish the austenic, pearlitic, bainitic, ferritic and martensitic microstructures previously described, in particular by microscopic observation on known means, for example a scanning electron microscope (SEM) or electron diffraction. backscattered (EBSD). This observation may, in known manner, be preceded by a chemical attack.

The characteristics relating to the operation of the installation 14, such as the intensity of the current in the devices 14 and 16, the temperature of the intermediate cold sources 48 and external 54, the flow rate of the external cold source 54 are in particular function the size of the element F, its diameter in the case of a wire, and the speed of movement of the element F. The values of these characteristics are within the reach of the person skilled in the art who will be able to determine by successive tests or by calculation.

FIG. 8 shows a second embodiment of a heat treatment unit for a reinforcing element for a tire. Elements similar to those represented in the first embodiment are designated by identical references.

In contrast to the first embodiment, the installation according to the second embodiment comprises several cooling devices 16 arranged in series downstream of the heating device 14.

The cooling devices 16 are interconnected by the circulation ducts 58 of the element F. These ducts also allow the circulation of the intermediate cold source 48. The cooling devices 16 are also interconnected by the circulation ducts 60 of the external cold source 54.

The invention is not limited to the embodiments described above.

In particular, it will be possible to consider simultaneously heat treating several reinforcing elements in the same cooling device. Thus, several reinforcing elements can scroll in the calorie extraction means which allows to further increase the total mass flow without increasing the size of the heat treatment plant.

It will also be possible to overlay several heat treatment plants one above the other to reduce the footprint.

The reinforcing element may be different from a wire, in particular different from a circular section wire.

It will be possible to implement the heat treatment method according to the invention in the context of a manufacturing process different from that previously described. In particular, it will be possible to use a manufacturing method comprising two drawing steps in a dry medium or two drawing steps in a humid medium.

The characteristics of the various embodiments described or envisaged above may also be combined provided that they are compatible with each other.

Note the great ease of operation of the process as well as the reduced size of the installation described above, unlike the method and installation of US4767472 in which the relatively low value of the speed of scroll of the installation is compensated by the simultaneous processing of several tens of wires which makes it possible to obtain a high total mass flow rate despite a mass flow rate per wire, that is to say a low unit mass flow rate. However, in US4767472, when the upstream storage coils are empty, it is necessary to abut the end of each wire to another wire from a full coil. It is therefore necessary to perform as much splicing as there are son, which makes the operation of the heat treatment plant of US4767472 relatively complex and tedious. In addition, it is necessary to have as many coils as there are son which makes the heat treatment plant of US4767472 relatively bulky.

Claims

1. Heat treatment plant for a reinforcement element (F) made of steel for a tire, characterized in that it comprises a heating device (14) for the reinforcement element (F) at a temperature (T1) greater than or equal to at the austenitization temperature of the steel comprising means (21) for supplying calories by joule effect through the reinforcing element (F).

2. Installation according to the preceding claim, wherein the means (21) for supplying calories by joule effect comprise two terminals (22, 24) electrically conductive.

3. Installation according to the preceding claim, wherein each electrically conductive terminal (22, 24) comprises an electrically conductive rotating pulley (23, 25).

4. Installation according to any one of the preceding claims, comprising, downstream of the device (14) for heating, a device (16) for cooling the reinforcing element (F).

5. Installation according to the preceding claim, wherein the cooling device (16) comprises means (32) for extracting calories from the reinforcing element (F).

6. Installation according to the preceding claim, wherein the means (32) for extracting calories comprise:

an enclosure (46) for scrolling the reinforcing element (F) containing an intermediate cold source (48) arranged between the reinforcing element (F) and an external cold source (54),

an enclosure (52) for circulating the external cold source (54) arranged around the scroll chamber (46) of the reinforcing element (F).

7. Installation according to claim 6, wherein the intermediate cold source (48) comprises a gas (50) of heat exchange.

8. Installation according to claim 6 or 7, wherein the external cold source (54) comprises a liquid (56) of heat exchange.

9. A method of heat treatment of a reinforcement element (F) steel for tire, characterized in that it comprises a step (200) of increasing the temperature of the reinforcing element (F) to a temperature (T1) greater than or equal to the austenitization temperature during at least a portion of which calories are supplied to the reinforcing element (F) by joule effect through the reinforcing element (F).

10. The method of claim 9, wherein the reinforcing element (F) is scrolled at an average speed of scrolling strictly greater than 40 m. min ^-1 , preferably greater than 90 m ^-1 min ^-1 , more preferably greater than or equal to 200 m. min ^-1 and even more preferably greater than or equal to 300 m min ^-1 .

1 1. A process according to claim 9 or 10, wherein the temperature of the reinforcing element (F) is increased at an average rate of increase of from 100 to 1000 ° Cs ^-1 , preferably 500 to 950 ° Cs ^"1 and more preferably 700 to 900 ° Cs ^-1 .